1. Field of the Invention
The present invention relates to image processing technology for processing image data.
2. Description of the Related Art
Image distortion can occur due to bending of the laser scanning line (laser distortion) caused by plane tilt of a polygon mirror that deflects the laser in an image forming apparatus such as a laser printer. Japanese Patent Application Laid-Open No. 2004-170755 discusses a distortion correction technology that corrects image distortion caused by laser distortion by shifting in the sub-scanning direction a pixel position of the image data at a position in the main-scanning direction according to the laser distortion. In Japanese Patent Application Laid-Open No. 2004-170755, laser distortion information representing a level of laser distortion in the sub-scanning direction is obtained at each position in the main-scanning direction by measuring a registration patch formed on a transfer belt with a detection sensor. Based on the laser distortion information, image data is formed so as to cancel out that laser distortion. When an image is output using this modified image data, the image distortion has been apparently corrected.
Japanese Patent Application Laid-Open No. 2004-170755 discusses a method for performing distortion correction based on laser distortion information obtained based on a measurement result by a detection sensor that detects a registration patch. Namely, Japanese Patent Application Laid-Open No. 2004-170755 discusses a detection sensor based distortion correction, which corrects laser distortion determined with the detection sensor.
However, Japanese Patent Application Laid-Open No. 2004-170755 does not discuss a distortion correction method that includes image distortion due to skew (geometric distortion) of the sheet on which the image is to be printed or skew caused by assembly error of the transfer belt that conveys the sheet, for example. Specifically, as illustrated in FIG. 11A, when the transfer belt and the sheet are skewed, for example, an image of a distortion-corrected straight line that is to be printed on a sheet is a straight line that is free from bending with respect to the detection sensor, in which bending due to laser distortion has been corrected. However, since the skew of the transfer belt and the sheet are not considered, the ultimately obtained image suffers from skew distortion with respect to the sheet.
Further, an image forming apparatus has been developed that uses two transfer belts, a primary transfer belt and a secondary transfer belt, in which laser distortion information is obtained by detecting with a detection sensor a registration patch formed on the primary transfer belt. This image forming apparatus is configured so that an image formed on the primary transfer belt is transferred onto the sheet that is conveyed by the secondary transfer belt. In this image forming apparatus, since a patch formed on the primary transfer belt is detected, the obtained laser distortion information does not consider a positional relationship, such as twisting of a position where the primary transfer belt and the secondary transfer belt are facing each other. Accordingly, the image that is printed on the sheet still suffers from skew distortion with respect to the sheet.